1. Field of the Invention
This invention relates in general to downhole drilling motors, and in particular to downhole drilling motors of the progressive cavity type.
2. Description of the Prior Art
Downhole drilling motors have been used for many years in the drilling of oil and gas wells. Usually, the bearing shaft of the motor, and the drill bit, will rotate with respect to the housing of the drilling motor. The housing is connected to the bottom of a conventional drill string, made up of drill collars and sections of drill pipe. At the surface, the drill string is connected to a kelly, which is mounted in the rotary table of a drilling rig.
Drilling fluid, or "mud," is pumped through the drill string to the bottom of the hole, and returns up the annulus between the drill string and the wall of the borehole. The mud cools the drilling tools and removes the cuttings from the hole. If the downhole drilling motor is hydraulic, the mud also supplies the hydraulic power to operate the motor.
One type of hydraulic downhole motor is the progressive cavity type, also known as the Moineau motor. This type of motor has a helical rotor within the cavity of a stator, and the stator is connected to a housing. As mud is pumped through the stator, the rotor is rotated. As the rotor rotates, it also gyrates, or orbits, in the reverse direction relative to its rotation. A universal connection must be used to connect the gyrating rotor to the bearing shaft of the motor, because the bearing shaft does not gyrate.
One type of connector has universal joints, which connect the ends of a straight rod to the rotor and to the bearing shaft. The universal joints take only torsional load, so a ball and race assembly is used to take the thrust load. Rubber boots are clamped over the universal joints to keep mud out of the ball and race assembly.
There are problems with rubber boot systems. Boots may loosen or come off, allowing mud to enter and wear out the ball and race assembly. The universal joints then must also take thrust loads, causing early failure of the universal joints. Most universal connectors also require oil reservoir systems to lubricate the ball race and the universal joints.
Other downhole motors have long, flexible connecting rods, which flex to compensate for the gyration of the rotor. Such connecting rods must be able to bear both thrust loads and torque loads. The rods must also be flexible enough to allow the eccentricity between the rotor and the bearing shaft, with a reasonably small side load. A one piece connecting rod, or a rod assembly, that has the required flexibility, also has torsional flexibility. That helps the motor run smoothly when dynamic external torques are encountered.
Occasionally excessive torque loads will cause a connecting rod to fail. A need existed for a connecting rod which is flexible, but which will not fail under excessive torque loads.